1. Field of the Invention
The present invention relates to a process for producing a grain-oriented electrical steel sheet having improved magnetic properties, more particularly, to a process during which a glass film having an improved insulating property, adhesivity, and appearance is uniformly formed on a steel sheet.
2. Description of the Related Arts
Ordinarily, a grain-oriented electrical steel sheet is produced by hot-rolling a starting material containing 4% or less of Si, annealing, cold-rolling once or twice or more with an intermediate annealing therebetween to obtain the final sheet thickness, decarburization-annealing in a wet atmosphere, applying, as an annealing separator, magnesia (MgO) in the form of a slurry by means of a coating roll, drying, and then finishing annealing.
In the grain-oriented electrical steel sheet, when determining the properties of the products and their value as goods, it is important that the film have excellent uniformity, thickness, adhesivity with the steel part, and insulating property, and an excellent tension effect. In the series of steps for producing a grain-oriented electrical steel sheet, MgO, which becomes the annealing separator, reacts with the oxide layer mainly composed of SiO.sub.2 formed during the decarburization annealing, to form a forsterite (Mg.sub.2 SiO.sub.4) film which is the glass film. The properties of MgO exert a great influence over the formation reaction of a glass film occurring in a coil. Ordinarily, the factors exerting an influence over the formation reaction of a glass film are the purity, grain size, activity, adhesivity, and the like of MgO. In addition, the advancing degree of hydration of MgO when slurried for preparing the annealing separator, aggregation degree of the MgO particles, application amount, and various additives have a great influence. Accordingly, to obtain an excellent film and magnetic properties, endeavors have been made to optimize the production conditions of MgO, i.e., formation condition of Mg(OH).sub.2 which is the raw material for forming MgO, and the baking condition when obtaining the MgO.
When MgO is applied on a steel sheet, it is suspended in water and slurried. Since the slurried MgO is applied on a steel sheet, a hydration reaction of MgO.fwdarw.Mg(OH).sub.2 partially occurs, so that a coil contains moisture, and the moisture between the sheets produces a high dew point and nonuniformity. This causes surface defects, such as frosting in the form of pinholes, gas marks, scale, discoloration, and the like due to excessive oxidation. As measures against this, endeavors have been made to control the characteristics of MgO by controlling the production conditions, or strengthening the cooling of the slurry when used, thereby lessening the moisture content between the sheets. Alternatively, a method has been disclosed for using MgO which is baked at a high temperature to suppress the hydration reaction (Japanese Unexamined Patent Publication No. 55-73823). Such an MgO, having a suppressed dehydration reaction, has a low reactivity with the SiO.sub.2 layer of oxide film, with the result that spangles, gas marks, adhesivity failure, and a decrease in the film tension may occur.
The influences of the reactivity of MgO and resultant moisture due to hydration of the MgO become greater as the size of a coil is increased, with the result that the glass film formation state is dispersed in the direction along the length and width of a coil. Accordingly, it is important to form a uniform glass film and enhance the magnetic properties by suppressing the resultant moisture content to an amount as small as possible, thereby eliminating the dispersion of the film-formation state.